Answer :
c=number of calzones sold
s=number of slices sold
c+s=86
10c+7s=770
alright
subsitute
c+s=86
minus s both sides
c=86-s
subsittute 86-s for c
10c+7s=770
10(86-s)+7s=770
860-10s+7s=770
860-3s=770
minus 860 both sides
-3s=-90
divide both sides by -3
s=30
subsitute back
c=86-s
c=86-30
c=56
56 calzones were sold
30 slices were sold
s=number of slices sold
c+s=86
10c+7s=770
alright
subsitute
c+s=86
minus s both sides
c=86-s
subsittute 86-s for c
10c+7s=770
10(86-s)+7s=770
860-10s+7s=770
860-3s=770
minus 860 both sides
-3s=-90
divide both sides by -3
s=30
subsitute back
c=86-s
c=86-30
c=56
56 calzones were sold
30 slices were sold